FIG. 1 shows an optical lithographic fabrication system 100 for defining features in a workpiece 120, in accordance with prior art. Typically the workpiece 120 comprises a semiconductor wafer (substrate), together with one or more layers of substances (not shown) located on a top major surface of the wafer.
More specifically, typically substantially monochromatic optical radiation of wavelength .lambda. is emitted by an optical source 106, such as a mercury lamp. This radiation propagates successively through an aperture in an opaque screen 105, an optical collimating lens or lens system 104, a patterned lithographic mask ("reticle") 103 having a pattern of features in the form of apertures (bright regions) in an opaque material, and an optical focusing lens or lens system 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the workpiece 120. Thus the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a development process (typically a wet development process), the material of the photoresist is removed or remains intact, respectively, at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101.
Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the workpiece 120 typically comprising a semiconductor wafer. That is to say, portions of the workpiece 120 are removed from the top surface of the workpiece 120 at areas underlying those where the photoresist layer 101 was removed by the development process but not at areas underlying those where the photoresist remains intact. Alternatively, instead of thus selectively etching the workpiece 120, impurity ions can be implanted into the workpiece 120 at areas underlying those where the photoresist layer was removed by the development process but not at areas underlying those where the photoresist remains. Thus, in any event, the pattern of the mask 103--i.e., each feature of the mask--is transferred to the workpiece 120 as is desired, for example, in the art of semiconductor integrated circuit fabrication.
As known in the art, the aperture in the opaque screen 105 is located on the focal plane of the collimating lens 104, and the indicated distances L1 and L2 satisfy in cases of a simple lens 102: 1/L1+1/L2=1/F, where F is the focal length of the lens 102.
In fabricating integrated circuits, it is desirable, for example, to have as many transistors per wafer as possible. Hence, it is desirable to make transistor sizes as small as possible. Similarly it is desirable to make as small as possible any other feature size, such as the feature size of a metallization stripe--i.e., its width--or of an aperture in an insulating layer that is to be filled with metal, for example, in order to form electrical connections between one level of metallization and another.
According to geometric optics, if it is desired to print on the photoresist layer 101 the corresponding feature having a width equal to W, a feature having a width equal to C is located on the mask (reticle) 103. Further, according to geometric optics, if this feature of width equal to C is a simple aperture in an opaque layer, then the "lateral magnification" m=W/C, where m=L2/L1. When diffraction effects become important, however, instead of a sharp black-white image, a diffraction pattern of the object feature C is formed on the photoresist layer 101, whereby the edges of the image become indistinct. Consequently, the resolution of the features of the reticle 103, as focused on the photoresist layer and transferred to the workpiece, deteriorates.
In prior art this diffraction problem has been alleviated by such techniques as using a mask having phase-shifting regions containing a pattern of phase-shifting features. The mask is then known as a "phase-shifting mask"--and hereinafter the mask 103 will therefore likewise be called "the phase-shifting mask 103." These phase-shifting features impart a phase shift .phi. to the optical beam of wavelength .lambda. propagating through the mask 103 relative to other features on the mask located on selected areas of the mask. Typically this phase shift .phi. is made equal to approximately .pi.(=180.degree.). The pattern of these phase-shifting features is typically located in a so-called "primary feature region" of the mask.
Phase-shifting masks can be fabricated with their phase-shifting features formed by "clear" areas and "attenuating" areas. A "clear area" is a transparent area--i.e., an area having an optical intensity transmission coefficient T=1, approximately, at the wavelength .lambda.. An "attenuating area" is a partially transparent area--i.e., an area in which T (at the wavelength .lambda.) advantageously is in the approximate range of 0.05 to 0.15, typically approximately 0.10. The phase shift .phi. is then the phase shift of the radiation of wavelength .lambda. propagating through the clear areas relative to the attenuating areas. Such a mask is known as an "attenuating phase-shifting mask." In addition, if desired, the primary feature region of such a mask can include opaque areas--that is, areas for which T&lt;0.01, approximately, at the wavelength .lambda..
Thus, in an attenuating phase-shifting mask, features such as that shown in the form of a simple aperture C in the mask 103 will become more complicated than just simple apertures and will include the pattern of the above-mentioned phase-shifting features (not shown in FIG. 1). More specifically, the primary feature region contains the abovementioned pattern of phase-shifting features whereby the images formed by them on the photoresist layer 101 correspond to device features of the workpiece 120 (such as selected areas of the workpiece 120 where impurity ions are implanted or selected areas of the workpiece 120 where portions of the workpiece 120 are removed).
As known in the art, a step-and repeat tool (not shown) is used in conjunction with a step-and repeat movement and exposure procedure for forming successive images on the photoresist layer 101. For this procedure, the workpiece 120 is subdivided into chip ("die") regions. Each such chip region typically is defined and encompassed by one resulting step-and-repeat position of the workpiece 120 (overlain by the photoresist layer 101). Each corresponding (i.e., overlying) chip region of the photoresist layer 101 is exposed in succession to the optical beam in the system 100. This exposure can be of the kind that exposes an entire chip region all at once to the incident optical beam (which has a sufficient cross section to encompass an entire chip region) or that scans the chip region with the incident optical beam (which does not have a sufficient cross section to encompass an entire chip region).
In order to align the mask 103 to the step-and-repeat tool (hereinafter, "stepper tool") for the step-and-repeat procedure, mask-to-stepper tool alignment marks (hereinafter "reticle alignment marks") are located on the mask 103 outside the primary feature region. Advantageously these reticle alignment marks (not shown in FIG. 1) comprise opaque areas. It is essential that these reticle alignment marks have edges that are self-aligned with respect to the edges of the primary feature region's attenuating areas. That is to say, it is essential that these reticle alignment marks and the attenuating areas of the primary feature region are formed during the same lithographic processing steps in which a single patterned resist layer defines both the edges of the reticle alignment marks and the edges of the attenuating areas of the primary feature region.
As further known in the art, in order to limit the optical radiation incident on the photoresist layer 101 to one chip region at a time, the stepper tool contains an opaquing shutter (hereinafter, "shutter blade"). In order to compensate for unavoidable positioning errors associated with locating this shutter blade, an opaque ring (not shown) is located on the mask itself. This opaque ring encircles the primary feature region. Advantageously, this opaque ring has an inside edge that is self-aligned with respect to, and borders the primary feature region.
In order to minimize the distance between chip regions (as defined by the step-and-repeat procedure), and hence to avoid wasting precious primary feature area on the workpiece 120, the inside edge of the opaque ring is self-aligned with respect to the features of the primary feature region. However, in the case of an attenuating phase-shifting mask, if (in the interest of economy) this ring is fabricated simultaneously with, and is fabricated with the same attenuating materials as those of, the attenuating areas (of the primary feature region), then this ring will thus transmit approximately 10 percent of the incident light intensity, instead of zero. Consequently, during the step-and-repeat exposure procedures, edge regions of the primary feature region will receive as much as approximately 20 percent (=2.times.10 percent) of the incident light intensity because of their having been exposed to the incident light during as many as two different step-and-repeat exposures. Corner regions will receive as much as approximately 40 percent (=4.times.10 percent) of the incident light intensity because of their having been exposed to the incident light on as many as four different step-and-repeat exposures. An unwanted optical background thus arises in the edge and corner regions of the primary feature regions. This background will cause deterioration of the definition of the features located in these edge and corner regions.
In addition to this background problem, the sharpness of definition (contrast ratio) of the images of the abovementioned reticle alignment marks are reduced when these alignment marks are formed (in the interest of economy) by attenuating areas simultaneously with the attenuating areas of the primary feature region, rather than by opaque areas. This reduction in contrast ratio can cause a deterioration of the accuracy of the alignment of the mask 103 relative to the stepper tool and hence ultimately to the workpiece 120.
Moreover, as mentioned above a phase-shifting feature in an attenuating phase-shifting mask is defined by two basic kinds of areas: (1) attenuating areas, and (2) clear areas. It would therefore be desirable to have a method of fabricating these two kinds of basic areas, together with opaque areas for the reticle alignment marks or for the opaque ring, or for both the reticle alignment marks and the opaque ring, in a self-aligned manner. That is to say, it would be desirable that the method can define the phase-shifting region, including the edges of its attenuating areas, during the same lithographic steps as the method defines the edges of the opaque ring or of the reticle alignment marks, or as the method defines the edges of both the opaque ring and the reticle alignment marks. In addition, if the primary feature region contains opaque areas, it would be desirable that the method can form these opaque areas during the same ltithographic steps as, and (if desired) in a self-aligned manner with respect to, the primary feature region's attenuating areas.